the principal cases of do-support, do does not normally occur when there is already an auxiliary or copula present; the auxiliary or copula is retained in the elliptical sentence: He is playing well, isn't he? I can cook pasta. Can you? You should get some sleep, and I should too. However, it is possible to use do as a pro-verb (see below section #Pro-verbs & Do-so Substitution even after auxiliaries in some dialects: Have you put the shelf up yet? I haven't done (or I haven't), but I will do (or I will). (However it is not normally used in this way as a to-infinitive: Have you put the shelf up? I plan to, rather than *I plan to do; or as a passive participle: Was it built? Yes, it was, not *Yes, it was done.) Pro-verbal uses of do are also found in the imperative: Please do. Don't! Pro-verbs and do-so substitution The phrases do so and do what for questions are pro-verb forms in English. They can be used as substitutes for verbs in x-bar theory grammar to test verb phrase completeness. Bare infinitives forms often are used in place of the missing pro-verb forms. Examples from Santorini and Kroch: Tests for constituenthood of a verb-phrase in X'-grammar The do so construction can be used to test if a verb-phrase is a constituent phrase in X'-grammar by substitution similarly to how other pro-forms can be used to test for noun-phrases, etc. In X-bar theory, the verb-phrase projects three bar-levels such as this: VP / \ ZP X' / \ X' YP | X | head With a simple sentence: S | VP / \ / \ / \ / \ NP \ / \ \ DP N' V' | | / \ The children / \ / \ V' PP / \ /_\ / \ with gusto V NP | /_\ ate the pizza Here again exemplified by Santorini and Kroch, do so substitution for testing constituent verb phrases in the above sample sentence: S | VP / \ / \ / \ / \ NP \ / \ \ DP N' V' | | / \ The children / \ / \ V' PP / \ /_\ / \ with gusto V NP | /_\ did so the pizza Use of do as main verb Apart from its uses as an auxiliary, the verb do (with its